JP2011-179541A discloses a control device for rotating a fluid pressure motor by utilizing a return fluid of a boom cylinder to rotate a motor generator by rotation force of the fluid pressure motor. In this control device, a regeneration control valve is provided in a process of a passage connecting a piston side chamber of the boom cylinder and a boom switching valve, and the regeneration control valve is connected to a regeneration flow passage connected to the fluid pressure motor.
In a case where the regeneration control valve is placed at a normal position, communication between the piston side chamber and the regeneration flow passage is blocked. In a case where the regeneration control valve is placed at a regeneration control position serving as a switching position, part of the return fluid is supplied to the regeneration flow passage as a regeneration flow rate. In a switching process where the regeneration control valve is switched from the normal position to the switching position, an opening degree of the regeneration flow passage is continuously changed, and the regeneration flow rate is controlled in accordance with the opening degree of the regeneration flow passage.
An opening degree of the regeneration control valve is controlled in accordance with an output signal of a controller. The controller controls the opening degree of the regeneration control valve in accordance with a spool stroke of the boom switching valve for controlling the boom cylinder. That is, the controller is configured so that the larger the spool stroke becomes, the larger the opening degree of the regeneration control valve becomes, thereby increasing the regeneration flow rate guided to the fluid pressure motor.
When a fluid is supplied to the fluid pressure motor, the fluid pressure motor is rotated and a motor generator linked with the fluid pressure motor is rotated to perform power generation. An assist pump to be rotated coaxially with the fluid pressure motor is linked with the motor generator, and the assist pump is driven and rotated by means of power of the motor generator.